An indirect fired water heater, such as the one disclosed in U.S. Pat. No. 4,313,400, for use in generating hot water for use in kitchens, bathrooms, laundries and the like in a building, typically includes a heat exchanger within a water tank and is in direct contact with the water therein. High temperature water is generated in a boiler and circulated through the water heater heat exchanger to raise the water temperature in the tank, thereby producing hot water. In addition to the water heater load, the boiler typically supplies other loads in the building, such as space heating load. When hot water in the water heater is drawn down and replaced by cold make-up water, a thermostat in the water heater demands high temperature water from the boiler. Since the output of the boiler is shared with other heating loads, the amount of high temperature water available to the water heater is relatively less. Thus, the recovery rate or the time required to heat up the water to the temperature set by the thermostat will be longer than when the boiler is dedicated to the water heater. Consequently, inconvenience due to lack of adequate amount of hot water may be experienced.
To alleviate this problem, a control device is needed that will shut off the other loads of the boiler when the water heater is calling for heat during a drawdown so that at least a substantial portion of the boiler output is directed to the water heater to provide it with a relatively faster recovery rate.